1. Field of the Invention
Generally, the present invention relates to equipment that is used for subsea oil and gas operations, and more particularly to methods that may be used to facilitate the retrieval and replacement of subsea oil and gas production and/or processing equipment.
2. Description of the Related Art
One of the most challenging activities associated with offshore oil and gas operations is the retrieval and/or replacement of equipment that may be positioned on or near the sea floor, such as subsea production and processing equipment and the like. As may be appreciated, subsea production and processing equipment, hereafter generally and collectively referred to as subsea equipment, may occasionally require routine maintenance or repair due to regular wear and tear, or due to the damage and/or failure of the subsea equipment that may be associated with unanticipated operational upsets or shutdowns, and the like. In such cases, operations must be performed to retrieve the subsea equipment from its location at the sea floor for repair, and to replace the subsea equipment so that production and/or processing operations may continue with substantially limited interruption.
In many applications, various cost and logistical design considerations may lead to configuring at least some subsea equipment components as part of one or more subsea production or processing equipment skid packages, generally referred to herein as subsea equipment packages or subsea equipment skid packages. For example, various mechanical equipment components, such as vessels, pumps, separators, compressors, and the like, may be combined in a common skid package with various interconnecting piping and flow control components, such as pipe, fittings, flanges, valves and the like. However, while skid packaging of subsea equipment generally provides many fabrication and handling benefits, it may present at least some challenges during hydrocarbon removal, depressurization, and retrieval of the equipment to the surface, as will be described below.
Depending on the size and complexity of a given subsea equipment skid package, the various equipment and piping components making up the skid package may contain many hundreds of gallons of hydrocarbons, or even more, during normal operation. In general, this volume of hydrocarbons in the subsea equipment skid package must be properly handled and/or contained during the equipment retrieval process so as to avoid an undesirable release of hydrocarbons to the surrounding subsea environment.
In many applications, subsea systems often operate in water depths of 5000 feet or greater, and under internal pressures in excess of 10,000 psi or more. It should be appreciated that while it may be technically feasible to shut in subsea equipment and retrieve it from those depths to the surface while maintaining the equipment under such high pressure, it can be difficult to safely handle and move the equipment package on and around an offshore platform or intervention vessel, as may be the case, while it is under such high pressure. Moreover, and depending on local regulatory requirements, it may not be permissible to move or transport such equipment and/or equipment skid packages while under internal pressure.
Yet another concern with subsea equipment is that problems can sometimes arise when flow through the equipment is stopped, for one reason or another, while the equipment is present in the subsea environment. For example, in some cases, flow through a given piece of subsea equipment may be intentionally stopped so that the equipment can be shut in and isolated for retrieval to the surface. In other cases, flow may inadvertently cease during inadvertent system shutdowns that occur as a result of operational upsets and/or equipment failures. Regardless of the reasons, when flow through the subsea equipment is stopped, hydrates and/or other undesirable hydrocarbon precipitates, such as asphaltenes, resins, paraffins, and the like, can sometimes form inside of the equipment. In such cases, the presence of any unwanted precipitates or hydrates can potentially foul the equipment and prevent a system restart after an inadvertent shut down, or they can complicate maintenance and/or repair efforts after the equipment has been retrieved to the surface. These issues must therefore generally be addressed during such times as when flow through the equipment ceases, such as by removal and/or neutralization of the constituents that may cause such problems.
In other cases, potentially damaging constituents, such as carbon dioxide (CO2) or hydrogen sulfide (H2S) and the like, may be present in solution in the liquid hydrocarbons that may be trapped inside of the equipment during shutdown. For example, hydrogen sulfide can potentially form sulfuric acid (H2SO4) in the presence of water, which may attack the materials of the some subsea equipment, particularly when flow through the equipment is stopped and the sulfuric acid may remain in contact with the wetted parts of the equipment for an extended period of time. Furthermore, it is well known that carbon dioxide may also be present in the trapped hydrocarbons, and can sometimes come out of solution and combine with any produced water that may be present in the equipment so as to form carbonic acid (H2CO3), which can also be damaging the materials that make up the wetted parts of the equipment during prolonged exposure. As with the above-described problems associated with hydrates and hydrocarbon precipitates, remedial measures are sometimes required to address such issues that are related to the various constituents that can cause material damage to wetted components when flow through the equipment is stopped.
Accordingly, there is a need to develop systems and equipment configurations that may be used to overcome, or at least mitigate, one or more of the above-described problems that may be associated with the retrieval and/or replacement of subsea oil and gas equipment.